These methods extend the rest package to support the cookie header and
header deletion.
The deletion is necessary to delete an existing authorization header if
cookie auth should be used.
This is a problem when syncing a file which just needed its modtime
set with dropbox which can't set the mod time of a file without
re-uploading it.
Before this change we would delete the file, then the server side move
would fail moving the file to the backup-dir because it no longer
existed.
After this change the destination file is moved to the backup-dir
instead of being deleted and the new file is uploaded.
Fixes#2134
